The present invention relates to a semiconductor device, specifically to an FET (Field Effect Transistor), a BT (Bipolar Transistor), or a CCD (Charge Coupled Device) having a semiconductor heterojunction, which is for use in various electronic circuits, such as integrated circuits, and in the optical communication, and a method of fabricating the semiconductor device.
Higher speed and higher performance of these devices are recently required in accordance with higher speeds and larger capacities of computer systems and communication systems.
To speed up a operation of a semiconductor device, it is necessary to increase the carrier density and carrier mobility. Conventionally the carrier mobility has been enhanced by using compound semiconductors with high carrier mobilities, or by using a modulation-doped structure with a heterojunction to thereby reduce the impurity scattering.
But in a modulation-doped structure using a heterojunction, carrier density is determined by a band offset in the heterojunction interface. Even in the most studied GaAs/AlGaAs heterojunction, the carrier density is about 10.sup.12 cm.sup.-2, which needs further improvement.
Even in this modulation-doped structure, the doped impurities near the heterojunction interface are random, and the carriers sense inhomogeneous potentials. Disadvantageously inhomogeneous potentials scatter the carriers, and the carrier mobility is accordingly deteriorated. The carrier scattering due to the random impurities is an unavoidable problem to the conventional GaAs/AlGaAs structure used in HEMTs (High Electron Mobility Transistors), in which carriers are not generated without higher dopant concentrations.